At its peak at about 1:45pm local time (one hour ago), the output got up to 23.9 GW. (Actually, I thought I saw it reach 24 GW at that time, but the replay isn’t showing it go above 23.9 GW.)
I’m sure an official number still needs to be confirmed, but a full 0.5 GW increase according to SMA’s site makes for a very safe conclusion that we have a new record. It is an estimate based on the output of thousands of SMA solar power systems spread across the country.

If 764MW of solar panel capacity produced at an annual capacity factor of 15%, and the average contract offered $500/MWh, production would be ~1% of Ontario's demand (with 1.4TWh) and be responsible for ~6.5% of the costs ($711 million out of ~$11 billion).

Production would not be distributed equally throughout the year.

I suspect the most productive months are the months of April-June.when Ontario's hydroelectric generators are most productive and demand is light; therefore I am not surprised that May set a record for wholesale market rates in Ontario (I am surprised estimates for June indicate it will shatter May's record highs).

Until there is reporting on solar production in Ontario, within a coherent, timely, accounting of the costs comprising the global adjustment, Ontario's ratepayers should assume the greatest share of the cost increases are due to solar generation, simply because it's the least transparently reported.

Additional notes

During the solar record in Germany, according to entsoe, the country was exporting:
~2500MW to the Netherlands
~100MW to Switzerland
~1650MW to Denmark
~100MW to Austria

The price, according to the European Energy Exchange, dropped below 10 euros/MWh during the hour, on it's way to ~5 euros/MWh between 2 and 3 pm.